Nuclear fuel rod assemblies generally include between 200 and 290 fuel rods mounted in a square array within a support skeleton. The support skeleton is formed from top and bottom nozzles which are interconnected to each other by sixteen to twenty-four uniformly arrayed thimble tubes. The top and bottom nozzles are eight to nine inches square, and the thimble tubes are about thirteen feet long, so that the overall shape of the fuel assembly is an elongated, rectangular prism. The fuel rods themselves are about twelve feet long. To equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes between seven and nine grids, each of which has a square array of open cells for receiving and spacing the fuel rods. The grids are usually formed from flat plates of interlocking sheet metal in an "eggcrate" configuration which lend compressive strength to the grids with a minimum of weight.
When the fuel assemblies are placed into operation, they are lowered into the reactor core by a crane, and the neutron-absorbing control rods (which are interspersed between the fuel rods) are slidably removed. Pressurized water is conducted through the bottom nozzles of the fuel assemblies to absorb uniformly the heat generated by the nuclear reaction that occurs between the fuel rods. Over a period of time, the nuclear fuel within the fuel rods becomes exhausted, thereby necessitating the removal of the spent fuel assembly from the nuclear core, and the disposal of its fuel rods in the spent fuel pool of the nuclear power plant facility. However, to make optimum use of the limited amount of storage space available in the spent fuel pool, the spent fuel assembly is first taken to a cask loading area of the pool for consolidation. The cask loading area of the pool is approximately forty feet deep, and is filled with water. The water shields from radiation the workers who typically stand on a deck located above the pool.
Once the spent fuel assembly is loaded into the cask loading shaft, the workers remove at least the top nozzle of the assembly, and use a gripping device (such as that disclosed in U.S. Pat. No. 4,651,400 by Shield and commonly assigned to Westinghouse Electric Corporation) to grip and withdraw the spend fuel rods from the fuel assembly in order to load these rods into a storage cannister. Because of the relatively low thermal output of spent fuel rods, empirical studies have shown that they may be packed in parallel contact with one another in a "triangular" array, which is the densest possible arrangement for a plurality of rod-like objects. In this arrangement, the axes of the rods define the corners of equilateral triangles. The triangular arrangement of spent fuel rods advantageously reduces the volume that these rods occupy within the fuel assembly by 50%. This allows the spent fuel pool to hold twice as many spent fuel rods.
Tooling systems for removing the fuel rods from a nuclear fuel rod assembly are known in the prior art. Such tooling systems are often used to remove the spent fuel rods from a fuel rod assembly so that they may be consolidated into a storage cannister, and ultimately placed in the spent fuel pool of the nuclear power plant facility. Such tooling systems typically include a rod gripping mechanism for selectively gripping and ungripping one or more fuel rods in a fuel rod assembly after the top nozzle of the assembly has been removed. The rod gripping mechanism is connected to a crane-like mechanism and operates by lifting the rods out of the assembly by applying a tensile or pulling force, and lowering them into a storage cannister. An example of such a fuel mechanism is disclosed in U.S. Pat. No. 4,651,400. While such fuel rod removal systems are commonly used in the context of spent fuel consolidation processes, they also may be used to remove the unspent fuel rods from a damaged fuel rod assembly during a reassembly-type repair.
Unfortunately, the performance of tooling systems that remove the fuel rods from their respective assemblies by gripping and pulling them out is not without shortcomings. For example, the gripping, raising and lowering of spent fuel rods from such assemblies is a slow and tedious process. While the water in the cask loading area of the pool affords an effective shield for the majority of radiation emanating from the fuel being consolidated, the workers on the deck still receive potentially hazardous radiation largely due to the length of time necessary to complete the consolidation operation. Additionally, the tensile forces applied to the spent fuel rods when they are forcefully pulled out of the grids of the fuel rods assembly can cause the relatively brittle outer tube of Zircaloy.RTM. cladding to break, thereby contaminating the water in the spent fuel pool with pellets of radioactive uranium oxide. Finally, the complete withdrawal of the spent fuel rods from the fuel assembly requires these rods to be hoisted upwardly within the spent fuel cask at least fifteen feet. If any of the gripper mechanisms holding the rods should slip at this juncture, one or more of the rods could fall to the bottom of the pool and break.
One system for removing fuel rods from a fuel rod assembly that is both faster and safer than prior art systems is disclosed in U.S. patent application Ser. No. 063,368, filed June 18, 1987 by Ellingson et al., assigned to Westinghouse Electric Corporation, the entire specification of which is incorporated herein by reference. This system is adaptable to any size and most types of fuel rod assemblies now in commercial use and is mechanically simple and reliable despite the differences in sizes of the individual fuel rods. It further minimizes breakage of fuel rods during fuel consolidation. This system removes the fuel rods solely by pushing them through the support skeleton of the fuel assembly after the top and bottom nozzles have been removed. To this end, this system includes a pushing assembly having a push rod for pushing a selected rod out of its respective grid cells. A rod catching device having rod receiving cells and spring clips prevents the pushed out rods from sliding completely through the fuel assembly and falling to the floor of the spent fuel pool.
Existing systems such as this remove the fuel rods in 1-4 hours, which is a substantial improvement over the prior art. This broad range of time is due to the unpredictability of applying the pushing operation to a given set of fuel assemblies. However, to achieve cost and radiation exposure objectives, it would be desirable if the removal operation were consistently performed in less than two hours. Additionally, in existing pushing removal systems, the push rod travels the length of the fuel assembly, approximately 160 inches. This is time consuming and results in the removal of the part of the push rod that contacts the fuel rods from the radiation-shielding water, thereby exposing workers to additional radiation. Finally, such pushing methods and systems may not work with Westinghouse.RTM. optimized fuel or EXXON.RTM. fuel because the pushing tool can snag on the grid dimples in these fuel systems.